The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the devices, systems and/or methods or the background. The disclosure of any reference cited herein is incorporated by reference.
Catalytic or combustible (flammable) gas sensors have been in use for many years to, for example, prevent accidents caused by the explosion of combustible or flammable gases. In general, combustible gas sensors operate by catalytic oxidation of combustible gases. As illustrated in FIGS. 1A and 1B, a conventional combustible gas sensor 10 typically includes an element such as a platinum element wire or coil 20 encased in a refractory (for example, alumina) bead 30, which is impregnated with a catalyst (for example, palladium or platinum) to form an active or sensing element, which is sometimes referred to as a pelement 40, pellistor, detector or sensing element. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987). Combustible gas sensors are also discussed generally in Firth, J. G. et al., Combustion and Flame 21, 303 (1973) and in Cullis, C. F., and Firth, J. G., Eds., Detection and Measurement of Hazardous Gases, Heinemann, Exeter, 29 (1981).
Bead 30 will react to phenomena other than catalytic oxidation that can change its output (i.e., anything that changes the energy balance on the bead) and thereby create errors in the measurement of combustible gas concentration. Among these phenomena are changes in ambient temperature, humidity and pressure.
To minimize the impact of secondary effects on sensor output, the rate of oxidation of the combustible gas may be measured in terms of the variation in resistance of sensing element or pelement 40 relative to a reference resistance embodied in an inactive, compensating element or pelement 50. The two resistances are typically part of a measurement circuit such as a Wheatstone bridge circuit as illustrated in FIG. 1C. The output or the voltage developed across the bridge circuit when a combustible gas is present provides a measure of the concentration of the combustible gas. The characteristics of compensating pelement 50 are typically matched as closely as possible with active or sensing pelement 40. Compensating pelement 50, however, typically either carries no catalyst or carries an inactivated/poisoned catalyst.
Active or sensing pelement 40 and compensating pelement 50 can, for example, be deployed within wells 60a and 60b of an explosion-proof housing 70 and can be separated from the surrounding environment by a flashback arrestor, for example, a porous metal frit 80. Porous metal frit 80 allows ambient gases to pass into housing 70 but prevents ignition of flammable gas in the surrounding environment by the hot elements. Such catalytic gas sensors are usually mounted in instruments which, in some cases, must be portable and, therefore, carry their own power supply. It is, therefore, desirable to minimize the power consumption of a catalytic gas sensor.
Electrical power dissipation of catalytic gas sensors as illustrated in FIGS. 1A through 1C is typically on the order of 250 to 700 mW. Further, the catalytic element or pelement and the compensating element or pelement are expensive to produce. Together, the pair typically accounts for well over half of the cost to manufacture the catalytic gas sensor. Further, the compensating element, which must closely match the sensing element in size and environmental responses, accounts for nearly half of the sensor power and half the cost of the sensing element/compensating element pair. Substantial research effort has been devoted to the development of low power combustible gas detectors and to reducing the costs of or eliminating the compensating element. U.S. Pat. No. 6,663,834, for example, discloses a combustible gas sensor in which the compensating element is replaced electronically by a thermistor network to compensate for changes in ambient temperature. The power requirements of the sensor of U.S. Pat. No. 6,663,834, however, remain relatively high.
Reducing the size of the sensing element wire and catalyst support bead of the sensing element can reduce the power requirements of a combustible gas sensor. Decreased wire diameter is generally associated with higher resistance and thus lower current/power to achieve a certain operating temperature. Moreover, reducing the size/volume of the sensing element/pelement reduces the effects of humidity and pressure changes on the sensor. In the past, sensors included elements made from wires of ample size and strength to support themselves. As the technology advanced over the years, efforts were made to reduce the power levels required to operate the sensors by, for example, reducing the size of the sensing element. Whereas early combustible gas sensors required more than a watt of power for operation, recently available combustible gas sensors have been able to operate in the 200-300 milliwatt range.
Reducing element size has, however, required the incorporation of some form of mechanical support for the smaller diameter, more fragile sensing elements and/or wires. Common mechanical supports have included various packing methods, or the use of a third support post.
Unfortunately, such mechanical supports draw or conduct heat away from the sensing (and/or compensating) element and thus result in higher power requirements to operate the element at a particular range of temperature.